Advances in antibody engineering have enabled the development of various antibody fragments featuring different pharmacokinetic and binding properties (Wu et al 2005, Wu et al 2008, Wu et al 2009). A minibody is an antibody format which features a smaller molecular weight (˜80 kD) than the full-length antibody while maintaining the bivalent binding property against an antigen (Hu et al 1996). Because of its smaller size, the minibody features faster clearance from the system and enhanced penetration when targeting tumor tissue. With the ability for strong targeting combined with rapid clearance, the minibody is an optimized antibody format that may be used for diagnostic imaging (Wu et al 2005). Since the discovery of the first minibody against the tumor-associated target CEA, many minibodies have been developed against different cancer targets for preclinical diagnostic imaging including human epidermal growth factor receptor-2 (HER2) in breast cancer, B-lymphocyte antigen CD20 in non-Hodgkins' lymphoma, and prostate stem cell antigen (PSCA) in prostate cancer (Hu et al 1996, Leyton et al 2008, Olafsen et al 2004, Olafsen et al 2009). For example, an 123I-labeled CEA minibody has been evaluated in the clinic for imaging patients with colorectal cancer by SPECT and similar studies have been performed with an 111In-DOTA labeled minibody (Wong et al 2004). The development of novel imaging agents is particularly critical for the diagnosis, management, and treatment of specific cancers which are poorly imaged with current technology such as prostate cancer.
The development of imaging agents for all types of cancer is needed to enable the targeting, staging, and monitoring of the disease. Current methods for diagnostic imaging of prostate cancer remain relatively inaccurate. With an estimated 234,460 new cases and 27,350 deaths in 2006, an imaging agent capable of accurately diagnosing, staging, and monitoring prostate cancer is needed (Olson et al 2007).
Prostate Specific Membrane Antigen (PSMA), a cell-surface biomarker that is associated with prostate cancer (Slovin 2005), is a single-pass Type II transmembrane protein possessing glutamate carboxypeptidase activity, although the functional role of PSMA is not well understood (Olson et al 2007). Expression of PSMA is relatively limited in normal tissues outside of the prostate including the brain, small intestines, liver, proximal kidney tubules, and salivary gland (Olson et al 2007).
PSMA expression in prostate cancer increases with tumor aggressiveness and is the highest in high-grade tumors, metastatic lesions, and androgen-independent disease (Olson et al 2007). Therefore, PSMA is a cancer biomarker that is a good candidate for targeting by an imaging agent. PSMA expression is also upregulated in the neovasculature of many non-prostatic solid tumors including lung, colon, breast, renal, liver and pancreatic carcinomas as well as sarcomas and melanoma (Olson et al 2007).
Full-length antibodies that target PSMA have been developed, some of which are in various stages of preclinical and clinical development (Olson et al 2007). PSMA was originally defined by a murine antibody (mAb), 7E11, which recognized an intracellular epitope of PSMA (Olson et al 2007). The 7E11 mAb was later developed into a FDA-approved SPECT imaging agent called PROSTASCINT imaging agent for the detection and imaging of prostate cancer in soft tissue (Olson et al 2007). However, since 7E11 recognizes an intracellular epitope, PROSTASCINT imaging agent is a relatively poor imaging agent which is limited to detecting necrotic tumor tissue (Olson et al 2007). Having the pharmacokinetic properties of a full-length antibody, PROSTASCINT imaging agent also requires a long period of time between injection and imaging (Olson et al 2007). Furthermore, PROSTASCINT imaging agent is a murine antibody which elicits strong immune responses that prevent multiple dosing (Olson et al 2007).
Another full-length antibody that targets PSMA, J591, was discovered and subsequently deimmunized, the deimmunized version known as huJ591 (Liu et al 1997, Bander et al 2003). The deimmunized huJ591 is an anti-human PSMA antibody that recognizes and binds an extracellular epitope on PSMA (Bander et al 2003). The huJ591 antibody is being developed as a potential radioimmunotherapy agent against prostate cancer. In Phase I trials, DOTA-conjugated huJ591 antibody labeled with gamma emitting isotopes Indium 111 and Lutetium 177 demonstrated excellent targeting to metastatic sites, no immunogenicity, and multiple doses were well tolerated (Bander et al 2003, Milowsky et al 2004, Bander et al 2005, Olson et al 2007). Beyond prostate cancer, Phase I studies with 111In-DOTA huJ591 demonstrated specific targeting of tumor neovasculature of advanced solid tumors (Milowsky et al 2007).